Ia [immune response associated] specificities were studied on germinal cells from C3H and 129 mice by using an AT.H and AT.L antiserum. By absorption experiments and immunofluorescence studies on sections of teses from mice of various ages, Ia specificities were detected on cells of the germinal line from primary spermatocyte to spermatozoa. Ia specificities were not detectable on gonocytes or spermatogonia.